The present invention relates to a built-in generator arranged within a hub member of a bicycle wheel.
Generally, for supplying electric power to a bicycle lamp, use is made of a generator or dynamo which is in the form of a separate attachment mounted on the bicycle frame or fork. In such an attachment system, the dynamo includes a roller which is urged against the side wall or outer circumference of the tire to transmit its rotation to the roller with the aid of friction therebetween, so as to drive the dynamo and generate electric power.
It is required to rotate the roller at high speeds in order to generate sufficient electric power by the dynamo. With the attachment system, the high speed rotation of the roller is accomplished by using a small diameter roller which is driven by a large diameter tire. In this case, however, the mechanical transmission efficiency is generally very low, usually on the order of 50%, owing to the frictional transmission between the tire made of a rubber and the roller made of a metal. Moreover, the transmission efficiency is greatly affected by contact conditions of the friction surfaces, such as wetting, muddiness or the like, mounted conditions of the dynamo, and distortion of the wheel. Therefore, the transmission efficiency often further lowers depending upon the contact condition of the friction surfaces.
In the attachment system, moreover, the dynamo is mounted on the bicycle frame or fork as a separate part so that the dynamo partly protrudes outwardly of the bicycle. Cyclist's leg or the like body parts, or other obstacles are likely to touch the protrusion of the dynamo so that the protrusion would interfere with cyclist's operation of the bicycle. Moreover, when the protrusion of the dynamo is hit by obstacles as upon falling down of the bicycle, the mounting of the dynamo may be distorted. Therefore, the mounting of the dynamo must be carefully corrected or adjusted, requiring time-consuming and troublesome operations.
As described above, the hitherto used attachment system suffers from various disadvantages, i.e. low efficiency, interference with operation, troublesome maintenance and the like. Accordingly, there has been a long demand for an improved generator for bicycle, which is small-sized and lightweight and operates with a high efficiency without interfering with the operation of the bicycle.
In view of such a demand, the applicant has proposed an improved generator as disclosed in non-prepublished Japanese Patent Application Nos. 1-202,915; 1-202,916; and 1-202,917. According to the proposed construction, the generator is of a built-in type and arranged within a hub member of a bicycle wheel. In more detail, there is provided in the hub member a generating coil unit fixed to an axle, a rotor integral with a magnet provided rotatably on the axle, and planetary gears as a speed increasing device provided around the axle for transmitting the rotation of the axle to the rotor at increased speeds.
With such an arrangement of built-in generator, the rotation of the wheel is transmitted to the rotor at an increased speed of about twenty-five times the rotational speed of the wheel with the aid of the speed increasing action of at least one planetary gear device. In this case, for example, when a bicycle with the wheel diameter of 26 inches runs at a speed of 20 km/h, the rotor is rotated at a speed of as high as 4,000 rpm.
In general, gears used in the planetary gear device of this kind are small in size and inferior in accuracy. Besides, gears need back-lashes so that positional relations between the gears are not necessarily correct. Moreover, axes of the gears are likely to be shifted. Therefore, when such planetary gears are driven at high speeds, noise tends to occur at engaging portions of gear teeth.
Furthermore, there can be rapid start and stop during running of the bicycle. As the bicycle equipped with the built-in generator is rapidly stopped during running while generating electric power, the rotor of the generator undergoes a rapid change from a rotating condition at a high speed to a stopped condition. In this case, the planetary gears are subjected to a substantial load due to the inertia mass of the rotor.
In general, as the planetary gears of this kind are preferably small and lightweight, resulting in insufficient strength. Thus, although the small gears serve sufficiently to operate in a usual power generating condition, there is often a risk of the gears being damaged due to a substantial load resulting from the rapid stopping or the like.